feat: model baseline context evaluation

[N4M3Z]

Summary

An OptimizeContext run against the Fable 5 baseline: drop or offload rules the model now follows without being told, and move always-on reference into the skills that own it.

• Rules: offload DeadVariables, NeverRefuseWork, PrepareDataOutsideLoops to rules/claude/claude-opus-4-7/ + claude-opus-4-8/ (parked until forge-cli#60 lands model-level qualifiers); slim BehavioralSteering to the rows the model can't infer; delete ScanRepository (VerifyClaims covers it). Stale-fixes to UseRTK, UseForge, KnownIssues, RuleStructure, NoTabs, MakefileFirst, SelfLearning, GitWorktrees, KeepFormatting, LineEndings, MemoryFiles.
• Rule into skill relocations (drop the always-on rule, fold it into the skill that already owns the topic): AgentStructure → BuildAgent, ArtifactComposition → BuildModule, ClaudePluginStructure + ClaudeCoworkLimitations → BuildPlugin, DeployManifest + DriftComparison + ProvenanceVerification → ProvenanceAudit, SecretScan + GitCryptUserData → SecretScan, WindowsConstraints + HookScripts + HookQuality → BuildHook, ValidationChain → VersionControl.
• CrossProviderAssembly → NoVendorLockIn: reframed around the point (no AI vendor is a single point of failure), "harness" not "provider".
• New: Deslop rule, OptimizeContext skill, config/models.yaml, MVPR-0004 writing up the method. MVPR-0003 + PROV-0005 amended.
• Skill body sweep: slims and stale-fixes across BashConventions, MarkdownConventions, BuildModule, BuildAgent, ArchitectureDecision, and more. BuildHook is past slimming and needs a rewrite, filed as forge-core#59.
• Qualifier dirs to exact model IDs: claude/opus-4-6 → claude/claude-opus-4-6.

Other sessions' work braided into the same jj change is held back for its own PRs: the jj/Entire tooling, the OnePurpose / UserConfig / VerifyDecisions / DataWorkingLayer / ScratchDirectory rules, a few unrelated skill adds.

Test plan

• forge validate --source . green at each batch through the run
• CI on the isolated branch
• post-merge: make install + /reload-skills